X-ray diffraction study and models of liquid ethylene at 106 K

X-ray diffraction data for liquid ethylene near the melting point are presented. The data are analyzed using scattering factors for –CH2 groups which permits extraction of the intermolecular carbon–carbon structure and distribution functions. Atom pair potentials derived from studies on crystalline hydrocarbons are used with the reference interaction site model (RISM) to predict structure and distribution functions for a dense ethylene-like fluid, and these are compared to the curves derived from the diffraction study. Site–site interaction potentials give an adequate description of liquid ethylene, provided that the attractive forces are averaged over the –CH2 groups with repulsive ’’bulges’’ at the hydrogen positions in the C2H4 molecule. Although –CH2 groups are ’’seen’’ by x rays as spherically symmetric, the hydrogen atoms have a significant effect on the carbon–carbon correlations in liquid ethylene near the triple point.